As the density of dynamic RAMs increases, it is necessary to use trench capacitors for charge storage. A dynamic memory cell using trench capacitors is disclosed in U.S. patent application Ser. No. 212,452 by Teng and filed on June 28, 1988, U.S. Pat. No. 4,958,206 which is incorporated herein by reference. A dynamic memory cell using a trench transistor is disclosed in U.S. patent application Ser. No. 026,356 by Teng, et al., filed Mar. 16, 1987, entitled "DRAM Cell and Method," U.S. Pat. No. 4,830,978, which is also incorporated by reference herein. Using a five volt supply, a 40 fF capacitor is necessary, resulting in a 200 fF capacitance. As memory cell size is decreased, however, a 3.3 volt, or lower, voltage supply will be used, thereby requiring a capacitance of at least 60 fF.
Present day trench capacitors are formed etching a trench in the substrate (the substrate acts a first plate of the capacitor), forming a dielectric layer along the sidewall of the trench, and refilling the trench with polysilicon (which acts as the second plate of the capacitor). The capacitance of present day trench capacitors may be increased by increasing the trench depth. However, it is difficult to achieve trenches with a true 90.degree. angle; therefore, for a given diameter, the depth which may be achieved is limited. For example, for a 1 micrometer trench, the trench depth is limited to approximately 6 micrometers for an sidewall angle of 85.degree. and approximately 9.5 micrometers for an sidewall angle of 87.degree.. More importantly, the increase in surface area between the plates is not significant when the depth is greater than 6 micrometers. For example, only a 15% increase in surface area (and hence capacitance) can be obtained when the depth is increased from 6 micrometers to 9 micrometers for a given angle.
A true vertical trench (90.degree.) may be possible. However, the profile of the trench may result in void formation during the polysilicon refill. The formation of voids within the trench may be fatal to the DRAM devices. The maximum trench angle which can be void-free is not known.
As memory devices become denser, the size of trench is used to form the capacitors must also decrease. Hence, it is necessary to provide more capacitor (because of the smaller volt supply) in a smaller trench (because of the size limitations).
Therefore, a need has arise in the industry for a trench capacitor having increased capacitance in a smaller area.